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In order of importance, left to right
PG > Twin Fang > Miasma > NB
PG, self explanatory.
Twin Fang since each lvl is the equivalent of about 80AP and its ratio is .55 Can also be used to last hit since your passive last's for 5 sec, if you time it right you should regain most if not all the mana back during those 5 sec's.
Miasma since its ratio is the worst out of all the skills and its debuff is twice as much per lvl compared to the MS buff from NB. Most often people will be in if for at least one second then it also lasts 2 sec when they are out. Thats as long as NB's.
NB best to keep this one low since its ratio is .9 and if need be can be used to keep the passive up.